1. Field of the invention
The present invention generally relates to automatic gain control (AGC) and, in particular, to a gain controller and method for a plurality of variable-gain amplifiers which are used in a radio apparatus such as a radio receiver or a radio transmitter.
2. Description of the Related Art
There has been disclosed a radio receiver including an AGC circuit which is designed to avoid distortion of an amplified signal and thereby improve a signal-to-noise ratio (S/N) in Japanese Patent examined Publication No. 3-24813. The radio receiver is provided with two radio-frequency (RF) variable gain amplifiers, a frequency converter, and an intermediate-frequency (IF) variable gain amplifier. The AGC circuit controls the respective gains of the RF amplifiers and the IF amplifier so as to keep the output signal level of the IF amplifier constant.
According to the conventional AGC circuit, the respective gains of the second RF amplifier and the IF amplifier are variable within different restricted gain ranges with respect to the output signal level of the IF amplifier. Referring to FIG. 4 of the above publication No. 3-24813, as the gain control signal Vc increases, the gains of the three amplifiers are sequentially reduced. More specifically, the gain of the IF amplifier is first reduced and then kept at a first reduced gain level. Since the gain of the IF amplifier is not reduced to below the first reduced gain level, in cases where the gain control signal Vc becomes further larger, the gain of the second RF amplifier is reduced and then kept at a second reduced gain level which is lower than the first reduced gain level. When the variable gain control signal Vc becomes furthermore larger, the gain of the first RF amplifier is finally reduced.